In a single-mode optical device with lateral confinement, it is generally important to favor the fundamental lateral optical mode over higher-order lateral modes. For example, a graph of the optical power produced by a diode laser as a function of its operating current is typically characterised by bends or ‘kinks’ joining essentially linear regions with different gradients. The kinks correspond to a phenomenon known as filamentation, whereby progressively higher order lateral modes of the laser cavity are excited with increasing current, accompanied by corresponding changes in the angle of maximum emission power. Such changes are undesirable because they degrade the coupling efficiency of a laser to an optical fiber. This limits the performance of single-mode diode lasers.
For example, high power, single emitter diode lasers operating at a wavelength of about 980 nm are used for pumping erbium-doped fiber amplifiers (EDFAs). Because the diameter of a single-mode EDFA is only a few microns, the coupling of output power from the laser to the EDFA is seriously compromised by angular shifts of as little as a few degrees. It is therefore desirable to operate such a high power pump laser in its fundamental transverse mode by ensuring that the kink power, i.e., the laser power at which filamentation first occurs, is as high as practically possible. The lifetime of a pump laser depends exponentially on its output power, and the closer a diode laser operates to the threshold power for instantaneous catastrophic optical damage (COD), the lower its lifetime. Commercial applications typically require lasers with operating lifetimes greater than about 105 hours. There is thus a need for high yield production of reliable 980 nanometer diode lasers with kink-free output powers in excess of about 200 milliwatts (mW).
It is desired, therefore, to provide a single-mode optical device that alleviates one or more of the above difficulties, or at least a useful alternative to existing single-mode optical devices.